Restoration of the balance of the free radicals nitric oxide (NO) and superoxide in the pulmonary vasculature may prevent life-threatening perioperative pulmonary hypertension (PH) in children with congenital heart defects (CHD) undergoing surgical correction of a left-to-right shunt. In rodent models of PH, intratracheal (IT) administration of R-190, a bifunctional NO donor and redox catalyst prodrug intended to correct free radical imbalance, selectively and profoundly reduces mean pulmonary arterial pressure (MPAP) without impacting systemic mean arterial pressure (MAP) or heart rate (HR). R-100, the metabolite of R-190, inhibits vascular remodeling. R-190 is dephosphorylated in vivo to form an intermediate, R-188, which is then hydrolyzed to form para-hydroxyphenylacetate (PHPA) and 3- nitratoproxyl-hydroxylamine. The freed PHPA detoxifies peroxynitrite, the reaction product of NO with superoxide anion, while the liberated 3-nitratoproxyl-hydroxylamine oxidizes to form R-100, a molecule serving as: a) a nitric oxide donor via its organic nitrate, and b) a broad-spectrum catalyst of reactive oxygen species degradation via its nitroxide moiety. The fusion of all the above functional domains into a single entity (R-190) assures that their multiple biologica activities are co-localized. The unique potency, selectivity, and sustained duration of effect justify development of R-190 for perioperative prevention of PH in CHD. Aim #1: Establish the pharmacodynamic (PD) profile of R-190 in a lamb model of PH induced by a thromboxane mimetic Juvenile lambs will undergo continuous infusion with the pulmonary-selective vasoconstrictor U-46619 to achieve a MPAP = 25-30 mmHg. R-190 (0, 1, 3, 10, 30 mg/kg nebulized per IT) will be compared to inhaled NO (iNO) for the amplitude and duration of its effect on SVR and PVR. Blood will be collected for correlation of PVR and SVR with the plasma concentrations of R-190 and its major metabolites. Aim #2: Determine the efficacy of R-190 in a lamb model of perioperative pulmonary hypertension induced by cardiopulmonary bypass (CPB) and surgical correction of a congenital left-t0- right shunt Juvenile lambs with PH (MPAP = ~25 mmHg) induced by in utero placement of a pulmonary - aortic window will undergo CPB and surgical closure of the arteriovenous connection. Directly following CPB, the effect of a single dose of nebulized IT R-190 or vehicle control will be compared to iNO (40 ppm) for the amplitude and duration of its effect on SVR and PVR. Aim #3: Establish the acute safety, stability, and tolerance of the aerosolized R-190 by IND-enabling toxicology and safety pharmacology studies. Lovelace Biomedical and Environmental Research Institute will carry out GLP-grade in vivo dose range- finding and 14 day repeat-dose toxicology investigations in juvenile rats and dogs, with a 14-day recovery period. RTX will then prepare and submit a full IND application to the FDA for a Phase 1a clinical dose- escalation study.